# std::any::type

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< cpp‎ | utility‎ | any

const std::type_info& type() const noexcept; |  | (since C++17)

Queries the contained type.

### Return value

The typeid of the contained value if instance is non-empty, otherwise typeid(void).

### Example

The example demonstrates `std::any` visitor idiom with ability to register new visitors at compile- and run-time.

Run this code

    #include <any>
    #include <functional>
    #include <iomanip>
    #include <iostream>
    #include <type_traits>
    #include <typeindex>
    #include <typeinfo>
    #include <unordered_map>
    #include <vector>

    template<class T, class F>
    inline std::pair<const std::type_index, std::function<void(const std::any&)>>
        to_any_visitor(const F& f)
    {
        return
        {
            std::type_index(typeid(T)),
            [g = f](std::any const& a)
            {
                if constexpr (std::is_void_v<T>)
                    g();
                else
                    g(std::any_cast<T const&>(a));
            }
        };
    }

    static std::unordered_map<std::type_index, std::function<void(const std::any&)>>
        any_visitor
    {
        to_any_visitor<void>([] { std::cout << "{}"; }),
        to_any_visitor<int>([](int x) { std::cout << x; }),
        to_any_visitor<unsigned>([](unsigned x) { std::cout << x; }),
        to_any_visitor<float>([](float x) { std::cout << x; }),
        to_any_visitor<double>([](double x) { std::cout << x; }),
        to_any_visitor<char const*>([](char const* s)
            { std::cout << std::quoted(s); }),
        // ... add more handlers for your types ...
    };

    inline void process(const std::any& a)
    {
        if (const auto it = any_visitor.find(std::type_index(a.type()));
            it != any_visitor.cend())
            it->second(a);
        else
            std::cout << "Unregistered type " << std::quoted(a.type().name());
    }

    template<class T, class F>
    inline void register_any_visitor(const F& f)
    {
        std::cout << "Register visitor for type "
                  << std::quoted(typeid(T).name()) << '\n';
        any_visitor.insert(to_any_visitor<T>(f));
    }

    int main()
    {
        std::vector<std::any> va{{}, 42, 123u, 3.14159f, 2.71828, "C++17"};

        for (int n{}; const std::any& a : va)
        {
            std::cout << (n++ ? ", " : "[");
            process(a);
        }
        std::cout << "]\n";

        process(std::any(0xFULL)); //< Unregistered type "y" (unsigned long long)
        std::cout << '\n';

        register_any_visitor<unsigned long long>([](auto x)
        {
            std::cout << std::hex << std::showbase << x;
        });

        process(std::any(0xFULL)); //< OK: 0xf
        std::cout << '\n';
    }

Possible output:

    [{}, 42, 123, 3.14159, 2.71828, "C++17"]
    Unregistered type "y"
    Register visitor for type "y"
    0xf

### See also

type_index (C++11) | wrapper around a `type_info` object, that can be used as index in associative and unordered associative containers (class)

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